Method of making thin film solar cell array

ABSTRACT

The present invention pertains to a thin film solar cell array that has an increased durability to high temperatures and high humidity. The thin film solar cell includes a transparent insulating substrate on which unit cells are placed in series. The rear electrodes of the unit cells are made of paste material containing conductive particles which may be applied by printing and baking at about 150° C. Further, the present invention achieves low contact resistance to the a-Si layer.

This is a division of application Ser. No. 07/268,904, filed Nov. 8,1988.

FIELD OF THE INVENTION

The present invention relates to a low cost thin film solar cell arraymade from unit cells which have a junction structure for photovoltaicconversion formed by use of amorphous silicon, and a electrode formed byprinting.

BACKGROUND OF THE INVENTION

Conventional thin film solar cell arrays which utilize amorphous silicon(hereinafter referred to as "a-Si") have a plurality of unit cellsconnected in series for the purpose of increasing the output voltage.Generally, each unit cell is composed of: a transparent electrode formedof ITO (indium-tin oxide) or SnO₂ and arranged on a transparentinsulating substrate, such as a glass substrate; an a-Si layer, having ap-i-n junction structure, and being composed of an approximately 200 Åthick p-type layer formed by glow-discharge decomposition of a gasmixture of silane, hydrocarbon such as acetylene, and diborane, anapproximately 0.5 μm thick non-doped layer, formed by glow-dischargedecomposition of silane gas, and, an approximately 500 Å thick n-typelayer, formed by glow-discharge decomposition of a gas mixture of silaneand phosphine; and, a rear electrode formed of an approximately 1 μmthick metal thin film.

The conventional series connection is established as follows. First, atransparent electrode film, an a-Si film and a rear electrode film aresuccessively formed on one and the same transparent insulatingsubstrate. The whole surface of the substrate is covered with therespective films. After each respective layer deposition, the respectivefilm is patterned to separate unit cells and to form connection portionsbetween adjacent cells. More specifically, after the transparentelectrode film is formed by electron beam evaporation or sputtering, thepatterning of the transparent electrodes is made by a printing method orphotolithography in which a resist pattern is formed by exposure using aphotomask for etching treatment. The patterning of the a-Si film is madeby a photolithography or laser-scribing method. The metal rear electrodefilm is formed by electron beam evaporation or sputtering method and ispatterned by photolithography.

Conventionally, an expensive apparatus for evaporation or sputtering isused. Additionally, photolithographic techniques are used to form therear electrodes of a thin metal film. Accordingly, it is difficult toreduce the number of manufacturing steps. Typically, in order to reducethe manufacturing costs, printing of the rear electrodes has beensuggested as a means to make it possible to perform simultaneously thefilm forming process and the patterning process.

In the case of a crystalline Si solar cell or a polycrystalline Si solarcell, the electrodes can be formed by coating the Si layer with a paste.The paste may be prepared by mixing Ag particles in epoxy resin andbaking the paste at 600-700° C. so that electrodes which come intoelectrical contact with the Si layer are formed. FIG. 2, curve 10,demonstrates the result where this technique is applied to an a-Si solarcell, using an illuminance of 200 lx. The fill factor is not larger than0.4. FIG. 2, curve 20, illustrates the characteristic of a solar cellfor an incident energy of 100 mW/cm². Heating of the a-Si solar cell to200 ° C. or more is impossible. Therefore, sintering, due to vaporizingof a high molecular weight resin, cannot be performed and a sufficientlylow contact resistance cannot be secured. Finally, conventional printedelectrodes suffer from poor resistance to humidity.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome the problems anddisadvantages of the prior art.

Another object of the present invention is a thin film solar cell inwhich the printed electrodes are formed by baking at about 150° C.

Another object of the present invention is a thin film solar cell havinga sufficiently low contact resistance to the a-Si layer.

A further object of the present invention is a thin film solar cellhaving increased durability to high temperatures and high humidity suchthat the printed electrodes can serve as rear electrodes.

To achieve the above objects and in accordance with the purpose of theinvention, as embodied and broadly described herein, the inventioncomprises a thin film solar cell array including a transparentinsulating substrate. Unit cells are arranged on the transparentinsulating substrate and are connected in series. Each unit cell iscomposed of a transparent electrode, an amorphous silicon layer with ajunction structure, and, a rear electrode. The unit cells are formed byutilizing selected resins, containing nickel particles as an electricalconductor and mulled with a silicon coupling agent. Other resinscontaining carbon particles as an electrical conductor and mulled with asilicon coupling agent are also used. Thereafter, the resins may besingly printed and baked. Alternatively, the two resins can be printedone over the other and then baked.

Low contact resistance to the a-Si layer can be achieved by printing therear electrode with a material obtained by adding a silicon couplingagent to a resin containing one member selected from the group ofamorphous carbon particles, graphite particles and nickel particles andthen baking such at about 150° C. In this way, the rear electrodeattains the characteristics of a large fill factor and excellentstability.

In an alternative embodiment, the rear electrode comprises first andsecond resin layers. The first resin layer contains electricallyconductive nickel particles and a silicon coupling agent mulled therein.The second resin layer, covering the first resin layer, containselectrically conductive amorphous carbon or graphite particles and asilicon coupling agent mulled therein. The rear electrode according tothis embodiment provides a stable electrical contact to the a-Si layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of thepresent invention and together with the description, serve to explainthe principles of the present invention.

FIG. 1 shows a sectional view of an embodiment of the solar cell arrayof the present invention;

FIG. 2 shows an output characteristic graph of an embodiment of thesolar cell array of the present invention and a conventional solar cell;

FIG. 3 shows a perspective view of another embodiment of the solar cellarray of the present invention;

FIGS. 4(a) through 4(d) show sectional views of a flow chart of aprocess for producing an additional embodiment of the solar cell arrayof the present invention; and,

FIG. 5 shows a graph of the laser output vs. the frequency of a laserpatterning process of the present invention in a machining mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a film of ITO (indium-tin oxide) or SnO₂ (tinoxide) is formed on a glass substrate 1 by electron beam evaporation orsputtering. Transparent electrodes 21, 22, 23, 24. . . are patterned byphotolithography. An a-Si layer having a p-i-n junction structure isformed in the same manner as described above. Patterns 31, 32, 33, 34. .. are formed by photolithography so that one end of each pattern coversa part of the corresponding space between adjacent transparentelectrodes 21, 22, 23, 24. . . Further, patters of metal electrodes 41,42, 43, 44. . . are formed by a printing method using phenol resincontaining nickel particles as a filler.

FIG. 3 shows a thin film solar cell array having a structure differentfrom that of FIG. 1. In FIG. 3, a-Si layer 3 is continuous so that it isnot separated by unit cells. Connection of the unit cells is made byoverlapping extensions or end portions of rear electrodes 41, 42, 43,44. . . with extensions or end portions of transparent electrodes 22,23, 24, 25. . . which are located off to one side with respect to thedirection of arrangement of the cells. Also the printed metal electrodesin the solar cell array having the aforementioned structure can serve asrear electrodes 41, 42, 43, 44. . . .

FIGS. 4(a) through 4(d) show the process of producing a solar cell arrayaccording to the present invention. The reference numerals uniformlyrepresent similar elements in each of FIGS. 1 and 4(a-d). As shown inFIG. 4(a), a transparent electrically conductive film of SnO₂ with thethickness of 1000-3000 Å is formed on a glass substrate 1 by sputtering.Transparent electrodes 21-25 are formed by a beam of a YAG laser lightwith a wavelength of about 1.06 μm and a spot diameter of 60 μm. Asshown in FIG. 4(b), an approximately 1 μm thick a-Si layer 3, having ap-i-n junction structure, is formed by plasma CVD as described abovewith reference to FIG. 1. As shown in FIG. 4(c), metal electrodepatterns 41, 42, 43, 44, 45. . . are applied and hardened by theaforementioned printing method. The width "w" of each of the overlappingportions 6 of the metal electrodes 41, 42, 43, 44 with the respectivetransparent electrodes 22, 23, 24, 25 is 30 μm. The distance "d" betweenadjacent metal electrodes (spaces 7) is approximately 50 μm.

From the side of the glass substrate 1, YAG laser light with awavelength of 0.53 μm and a spot diameter of about 50-80 μm is focusedon the overlapping portions 6 where the metal electrodes overlap withthe transparent electrodes. The irradiation of the light spot is carriedout while the center point of the light spot is moved along the spacesbetween the side end surfaces of the adjacent metal electrodes 41, 42,43, 44, 45. . . As a result, the a-Si layer 3 is cut at the lowerportions 8 of the spaces 7 between adjacent metal electrodes.Accordingly, the a-Si layer 3 is separated into segments, 31, 32, 33,34, 35. . . Connection areas 61, 62, 63, 64. . . for connecting theopposite electrodes to each other are made from overlapping portions 6which are finely crystallized where they contact the cut portions 8.

As shown in FIG. 5, the patterning and crystallization with laser lightcan be made when the laser output is within the region A of themachining mode. When the Q-frequency is less than or equal to 3 kHz, theprinted metal electrodes are often injured. Therefore, it is necessaryto keep the frequency of the laser light between 3 kHz and 8 kHz.Further, it is necessary to adjust the spot diameter so that adjacentmetal electrodes are not affected by a large laser spot.

According to the present invention, an electrically conductive filler,such as Ni or carbon particles, is mixed in a resin such as a phenolresin, melamine-alkyd resin, or the like. The resin is printed on asurface to which a silicon coupling agent has been previously applied.Alternatively, resin mulled with such a silicon coupling agent may beprinted on a surface. As a result, printed rear electrodes, capable ofcontacting the a-Si layers with low resistance, can be prepared bybaking the resin at low temperatures. Accordingly, a low-cost solar cellhaving a large fill factor and good durability against heat and humiditycan be prepared. Further, by adding graphite particles to amorphouscarbon particles or by adding an oxide of Sn, such as ITO and SnO₂, andby regulating the respective particle sizes, characteristics includingthe stability of the solar cell array can be improved.

Further, deterioration of the solar cell array can be prevented withoutthe application of protective films by disposing two resins one over theother. The resins are selected from phenol resin, melamine alkyd resin,or the like. One of the resins contains nickel particles as anelectrically conductive filler and is mulled with a silicon couplingagent. The other resin contains carbon particles as an electricalconductor and is mulled with a silicon coupling agent. Further, byaddition of an additive, such as ITO and SnO₂, to the two resin layers,the characteristics of the solar cell array are improved.

The First Embodiment

The initial characteristics of the thin film solar cell of the firstembodiment is shown in FIG. 2 as curve 30. The initial fill factor ofthe thin film solar cell was 0.60. The test results wherein the solarcell was left in high-temperature and high-humidity conditions of 60° C.and 90%, respectively, show that the solar cell had a fill factor whichwas lowered to about 0.50 after 40 hours and recovered to approximately0.55 after about 500 hours.

The phenol resin used for the first embodiment was prepared via thereaction of phenol, formalin, rosin and oil and the esterificationthereof with glycerol. The particle size of the nickel particles usedwas adjusted to be 1 to 10 mm by a ball mill or the like. The nickelparticles were mixed in the phenol resin in the proportion of 1-5 partsby weight of nickel particles per one part of the resin. The surfaces ofa-Si layers 31, 32, 33, 34. . . were soaked in a silicon coupling agent,such as q-aminopropyltriethoxysilane (tradename: TSL8331, TOSHIBASILICONE CO, Co., Ltd.), N-(b-aminoethyl)- q-aminopropyl-triethoxysilane(tradename: TSL8340, TOSHIBA SILICONE CO., Ltd.),q-glycidoxypropyltriethoxysilane (tradename: TSL8350, TOSHIBA SILICONECo., Ltd.).

The surfaces were then dried and baked. The viscosity of the phenolresin containing nickel particles was adjusted by use of an ethyleneglycol series or diethylene glycol series solvent. The patterns of rearelectrodes 41, 42, 43, 44. . . were printed with the phenol resin by useof a screen mask having a mesh size of 120-180 so that one end of eachrear electrode covered a part of a corresponding space between adjacenta-Si layers 31, 32, 33, 34. . . and touched an end portion of thenext-cell transparent electrode 22, 23, 24, 25. . . . The patterns werehardened at 120-189° C. for 20-60 minutes.

The Second Embodiment

The initial fill factor of the thin film solar cell of the secondembodiment was 0.61. The solar cell was left in the high-temperature andhigh-humidity conditions of 60° C. and 90%, respectively. As a result,the fill factor of the solar cell was 0.60 after about 500 hours or inother words the fill factor was lowered by about 2%. The particle sizeof Ni after the roll dispersion was 3 to 20 mm. The results indicatethat if the particle size was 5 to 15 mm, the fill factor would beimproved by about 0.01. In short, it was found that the preferredparticle size was within a range between about 30% of the film thicknessand about two times the film thickness. The more preferred particle sizewas a range between 0.5 times and 1.5 times the film thickness.

In the second embodiment, printed rear electrodes were prepared asfollows. The particle size of nickel particles was adjusted to 5 to 10mm by a ball mill or the like. The nickel particles were mixed in phenolresin, as defined in the first embodiment, in the proportion of 1-5parts by weight of nickel particles per one part of the resin. Then0.1-5% by weight of a silicon coupling agent was added to the resin. Theviscosity of the resulting resin was adjusted to 100-400 poise by use ofa solvent as defined in the first embodiment. The particle size wasadjusted by roll dispersion treatment in which the resin was passedbetween the rolls. The resin was applied to form a 10-20 mm thick filmby use of a screen mask of 180-250 mesh size. The film was hardened at120-180° C. for 20-60 minutes.

The Third Embodiment

The initial fill factor of the thin film solar cell of the thirdembodiment was 0.61. The test in which the solar cell was left inhigh-temperature and high-humidity conditions of 60° C. and 90%,respectively, showed very little change in the fill factor after 500hours.

In the third embodiment, a mixture of phenol resin, as defined in thefirst embodiment, and alkyd resin in the proportion 4:6 was used as theresin. The alkyd resin used was prepared by adding drying oil, such aslinseed oil, tung oil, soybean oil or the like, dehydrated castor oil orconstituent fatty acid thereof to a series of phthalic anhydride andglycerol. Other treatments, such as roll dispersion treatment, were madein accordance with the second embodiment.

The Fourth Embodiment

The initial fill factor of the thin film solar cell of the fourthembodiment was 0.59. The test results wherein the solar cell was left inhigh-temperature and high-humidity conditions of 60° C. and 90%,respectively, indicated that the fill factor was lowered to 0.57 after500 hours, i.e., the fill factor hardly deteriorated.

The fourth embodiment involved a solar cell, produced in the same manneras the third embodiment, except that a mixture of the phenol-modifiedalkyd resin, as defined in the third embodiment, and epoxy resin wereused. The epoxy resin used, for example, was prepared by interactingepichlorhydrin to bisphenol A made from phenol and acetone.

When the solar cell was produced in the same manner except that ITOparticles divided into 3 μm segments were added in the proportion of20-60% to the Ni filler of the resin, the initial fill factor producedwas 0.61. The results of the test in which the solar cell was left inhigh-temperature and high-humidity conditions of 60° C. and 90%,respectively, showed the fill factor was 0.58 after 500 hours.

When the solar cell wa produced in the same manner with the exceptionthat the ITO particles, which were used as a filling material, werereplaced by SnO₂ particles, there were no characteristic differences inthe solar cell.

The Fifth Embodiment

The initial fill factor of the thin film solar cell of the fifthembodiment was 0.60. The results of the test in which the solar cell wasleft in high-temperature and high-humidity conditions of 60° C. and 90%,respectively, indicated that the fill factor of the solar cell barelychanged after 500 hours.

In the fifth embodiment, a mixture of 4-6 moles of formaldehyde andbutanol were added to 1 mole of melamine. While the pH value of theresulting mixture was adjusted to 6-8 by addition of ammonia, amethylolizing reaction was conducted at 90-100° C. for 30 minutes.Thereafter, a small quantity of butyl phosphate was added to cause abutyletherification reaction for about an hour. The pH values wereadjusted to 4-5. Xylol was added to remove generated water and butanol.The product was diluted with a solvent selected from xylol, butanol anda mixture thereof to prepare a melamine resin solution. Further, 30-45%of short-oil-length alkyd resin was mixed with the melamine resin. Theshort-oil-length alkyd resin was prepared by adding drying oil, such aslinseed oil, tung oil, soybean oil and the like, dehydrated castor oilor constituent fatty acid to a series of phthalic anhydride andglycerol.

Nickel particles, divided into 10 μm, were mixed in the melamine-alkydresin. Then, 0.1-5% by weight of a silicon coupling agent was added.After roll dispersion treatment, the resin was applied to form a 10-20μm thick film by use of a screen mask of 180-250 mesh size.

In the case where a solar cell was produced in the same manner asdescribed above with the exception that ITO or SnO₂ particles were addedto the Ni filler of the resin, the initial fill factor of the solar cellslightly improved to 0.61.

Where the metal electrodes were printed by baking at 150 ° C. for 60minutes using the same material as in the second embodiment, the fillfactor of the thin film solar cell produced was not less than 0.65 underirradiation conditions of 10 mW/cm² and 100mW/cm². Where the Ni particlesize was regulated at 5 to 10 μm by the roll dispersion treatment, thefill factor slightly improved to 0.67. The deterioration of the fillfactor as a result of the test in which the solar cell was left inhigh-temperature and high-humidity conditions of 60° C. and 90%,respectively, was no more than 3-5%. The process of positioning the a-Silayer patterns was not required. Accordingly, spaces for the positioningprocess were not required. Consequently, in the case where the unitcells were arranged at pitches of 5 mm, the width of an ineffectiveportion between adjacent elements was reduced from 250 μm to 150 μm toimprove the effective area ratio to 91%.

The Sixth Embodiment

The characteristic of the thin film solar cell of the sixth embodimentis shown in FIG. 2 at curve 30. The initial fill factor of the thin filmsolar cell was 0.60. The test results wherein the solar cell was left inhigh-temperature and high-humidity conditions of 60° C. and 90%,respectively, indicated that the solar cell fill factor was lowered toapproximately 0.50 after about 40 hours but recovered to approximately0.55 after 500 hours.

The sixth embodiment involves the use of amorphous carbon as a filler.The sixth embodiment follows the same initial structure of the firstembodiment as depicted in FIG. 1 and explained previously.

The phenol resin used was prepared by the reaction of phenol, formalin,rosin and oil and the esterification thereof with glycerol. The particlesize of the carbon particles was adjusted to about 1 μm by a ball millor the like. The ratio of filler to resin was about 60-120%. Thesurfaces of the a-Si layers 31, 32, 33, 34. . . were soaked in a siliconcoupling agent, such as γaminopropyltriethoxysilane (tradename: TSL8331,TOSHIBA SILICONE Co., Ltd.),N-(β-aminoethyl)-γ-aminopropyltriethoxysilane (tradename: TSL8340,TOSHIBA SILICONE Co., Ltd.), γ-glycidoxypropyltriethoxysilane(tradename: TSL8350, TOSHIBA SILICONE Co., Ltd.) and the like for 1-15minutes. Then the surfaces were dried and baked.

The viscosity of the phenol resin containing carbon particles wasadjusted by use of an ethylene glycol series or diethylene glycol seriessolvent. Patterns of rear electrodes 41, 42, 43, 44. . . were printedwith the phenol resin by the use of a screen mask of 120-180 mesh sizeso that one end of each rear electrode covered a part of a correspondingspace between adjacent a-Si layers 31, 32, 33, 34. . . and touched anend portion of the next-cell transparent electrode 22, 23, 24. . . .Finally, the patterns were hardened at 120-180° C. for 20-60 minutes.

The results did not appear to change when graphite having a particlesize of 10 μm was used as a filler and mixed with amorphous carbon suchthat the ratio of amorphous carbon to graphite was in the range 0.5-2.

The Seventh Embodiment

The initial fill factor of the thin film solar cell of the seventhembodiment was 0.56. The results wherein the solar cell was left inhigh-temperature and high-humidity conditions of 60° C. and 90%,respectively, indicated that the fill factor was reduced to 0.51 after20-50 hours but recovered to 0.56 after 500 hours.

The seventh embodiment relates to a mixture of phenol resin, as definedin the sixth embodiment, and alkyd resin in the proportion of from 4:6to 2:8. The alkyd resin used was prepared by adding drying oil, such aslinseed oil, tung oil, soybean oil and the like, dehydrated castor oilor constituent fatty acid thereof to a series of phthalic anhydride andglycerol. Additional treatment was made in the same manner as in thesixth embodiment.

The Eight Embodiment

The initial fill factor of the solar cell of the eighth embodiment was0.61. The test results wherein the solar cell was left inhigh-temperature and high-humidity conditions of 60° C. and 90%,respectively, indicated that the fill factor was reduced to 0.48 after20-50 hours but recovered to 0.55 after 500 hours.

The solar cell of the eighth embodiment was produced in the same manneras that of the third embodiment, except that a mixture of thephenol-modified alkyd resin, as defined in the seventh embodiment, andepoxy resin was used. For example, the epoxy resin used was prepared byinteracting epichlorhydrin to bisphenol A made from phenol and acetone.Amorphous carbon and graphite, in the proportion of 0.5-2, were mixed inthe resin to achieve a ratio of resin to filler of 0.3-2.0. Then asilicon coupling agent, as defined in the sixth embodiment, was mixedand mulled with the resin. Then roll dispersion treatment was applied tothe resin. The preferred ratio of resin to filler was determined to be0.5-1.5.

The Ninth Embodiment

The initial fill factor of the solar cell of the ninth embodiment was0.60. The test results wherein the solar cell was left inhigh-temperature and high-humidity conditions of 60° C. and 90%,respectively, indicated that the fill factor was reduced to 0.52 after50 hours but recovered to 0.60 after 500 hours.

In the ninth embodiment, particles of ITO (indium-tin oxide), ofparticle size in the range of 2-15 μm, were added to a filler ofamorphous carbon and graphite, as defined in the eighth embodiment, at aproportion of 20-80% per total amount of carbon and graphite. Furtherconditions employed, such as the mulling condition of the silanecoupling agent and the condition of roll dispersion treatment, weresimilar to those of the eighth embodiment. The thickness of the coatingfilm was 10-20 μm.

When the particle size of ITO was regulated at 5 to 15 μm, the fillfactor improved by about 0.01. The results did not change when the solarcell array was produced in the same manner as described above exceptthat the additive particles of ITO were replaced by particles of SnO₂(tin oxide).

The aforementioned results indicate that characteristics of the solarcell can be improved by the addition of electrically conductiveparticles having a particle size 0.2-1.5 times the thickness of thecoating film, and preferably in the range of 0.5-1.0 times as thick.

The Tenth Embodiment

The initial fill factor of the thin film solar cell of the tenthembodiment was 0.59. The results wherein the solar cell was left inhigh-temperature and high-humidity conditions of 60° C. and 90%,respectively, indicated that the fill factor was reduced toapproximately 0.57 but recovered to 0.58 after 500 hours.

The tenth embodiment follows the initial steps of the fifth embodimentdescribed herein, first paragraph.

Additionally, ITO or SnO₂ for doping was formed into a particle size ofabout 10 μm. A filler consisting of amorphous carbon particles, graphiteparticles and the doping particles in the proportions 4:6:4 was mixed inthe resin so that the ratio of filler to resin became 0.5-2.0, orpreferably, 1.0-1.5. Thereafter, 0.1-5% by weight of a silane couplingagent was added. After roll dispersion treatment, the resin was used forthe production of printed electrodes.

The Eleventh Embodiment

The eleventh embodiment employed printed rear electrodes formed byprinting two resins one over the other and subsequently burning them.One of the resins contained carbon particles mulled with a siliconcoupling agent. The other resin contained nickel particles mulled with asilicon coupling agent.

Referring to FIG. 1, patterns of metal electrodes 41, 42, 43, 44. . .were formed by a printing method through lamination of phenol resin,containing nickel particles as a filler, and phenol resin, containingamorphous carbon particles as a filler. The phenol resin used wasprepared by the reaction of phenol, formalin, rosin and oil, and theesterification thereof with glycerol. The phenol resin may be used incombination with alkyd resin so that the proportions of phenol to alkydis from 4:6 to 2:8. The alkyd resin used was prepared by adding dryingoil to a series of phthalic anhydride and glycerol. The drying oil mayinclude linseed oil, tung oil, soybean oil and dehydrated castor oil orconstituent fatty acid thereof.

The particle size of the nickel particles used was adjusted to about5-10 μm by a ball mill or the like. The nickel particles were mixed inthe phenol resin in the proportion of 1-5 parts by weight of nickelparticles per one part of the resin. Further, 0.1-5% by weight of asilicon coupling agent, such as γ-aminopropyltriethoxysilane (tradename:TSL8331, TOSHIBA SILICONE Co., Ltd.) was added to the resin. Theviscosity of the resulting resin was adjusted to 100-400 poise by use ofan ethylene glycol series or diethylene glycol series solvent. Theparticle size was adjusted by roll dispersion treatment in which theresin was passed between rolls. The resin was applied to form a 10-20 μmthick film by use of a screen mask of 180-250 mesh size. Then, the filmwas hardened at 120-180° C. for 20-60 minutes.

The initial fill factor of the solar cell of the eleventh embodiment was0.61. The test results wherein the solar cell was left in thehigh-temperature and high-humidity conditions of 60° C. and 90%,respectively, indicated that the fill factor was reduced to 0.60 after500 hours or in other words the fill factor was lowered by about 2%after 500 hours. The particles size of Ni after the roll dispersion was3 to 20 μm.

Epoxy resin was mixed with the phenol-modified alkyd resin as definedabove to prepare a mixed resin. Amorphous carbon and graphite, in theproportion of 0.5-2, were mixed in the resin to produce a ratio of resinto filler of 0.3-2.0, or preferably, 0.5-1.5. Particles of ITO or SnO₂,of particle size of about 5-10 μm, were added to the resin in aproportion of 20-80% per the total amount of amorphous carbon andgraphite. Thereafter, 0.1-5% by weight of a silicon coupling agent wasadded. After roll dispersion treatment, the resin was printed on thefilm containing Ni as a filler and was baked at 120-180° C. for 20-60minutes.

As a result, the thin film solar cell produced had the initialcharacteristics as shown in FIG. 2 by curve 30. The initial fill factorof the thin film solar cell was 0.66 in 200 lx and was 0.63 in 100mW/cm². The initial fill factor was little changed underhigh-temperature and high-humidity conditions of 60° C. and 90%,respectively. The fill factor values after 500 hours were 0.65 and 0.61.

As a comparative example, the resin containing Ni as a filler wasprinted but was not baked to form a first resin layer. After

the first resin layer was left at room temperature for 10-30 minutes, asecond resin layer containing carbon as a filler was printed and baked.The fill factor of the solar cell produced was slightly improved. Theinitial fill factor was 0.67 in 200 lx and was 0.65 in 100 mW/cm².

the Twelfth Embodiment

The fill factor of the solar cell of the twelfth embodiment was 0.65 in200 lx and was 0.61 in 100 mW/cm². The initial factor barely changedunder high-temperature and high-humidity conditions of 60° C. and 90%,respectively. The values of the fill factor after 500 hours remainedconstant.

The twelfth embodiment employs the second resin layer of the eleventhembodiment formed of melamine-alkyd resin. The melamine-alkyd resin maybe prepared as follows. A mixture of 4-6 moles of formaldehyde andbutanol were added to 1 mole of melamine. While the pH value of theresulting mixture was adjusted to 6-8 by addition of ammonia, amethylolizing reaction was conducted at 90-100° C. for 30 minutes. Asmall quantity of butyl phosphate was added to undergo abutyletherification reaction for about an hour while the pH values wereadjusted to 4-5. The xylol was added to removed generated water andbutanol. The product was diluted with a solvent selected from xylol,butanol and the mixture thereof to prepare a melamine resin solution.Thereafter, 30-40% of short-oil-length alkyd resin was mixed with themelamine resin. The short-oil-length alkyd resin was prepared by addingdrying oil, such as linseed oil, tung oil, soybean oil and the like,dehydrated castor oil or constituent fatty acid to a series of phthalicanhydride and glycerol. The ratio of melamine to alkyd was from 20:80 to45:35. Particles of ITO or SnO₂, of particle size of about 5-10 μm, wereadded to the resin in the proportion of 20-80% per the total amount ofamorphous carbon and graphite. Thereafter, 0.1-5% by weight of a siliconcoupling agent was added. After roll dispersion treatment, the resin wasapplied to form a second resin layer.

The Thirteenth Embodiment

In the thirteenth embodiment, a solar cell was prepared in the samemanner as in the eleventh embodiment, except that fine powder of ITO orSnO₂ with a particle size of 5-10 μm was added to the resin of the firstresin layer in the proportion of 20-60% per the amount of Ni filler. Asa result, the fill factor of the solar cell was 0.67 in 100 mW/cm².

Other embodiments of the present invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the disclosed invention. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being represented by the following claims

What is claimed is:
 1. A method of producing a thin film solar cell,comprising the steps of:forming a transparent electrically conductivefilm on a transparent insulating substrate; forming said transparentelectrically conductive film into a pattern of transparent electrodes;forming an a-Si layer having a p-i-n junction structure on saidtransparent insulating substrate through said transparent electrodes;coating and hardening a plurality of metal electrode patterns on saida-Si layer by a printing method; and focussing and applying a laserlight beam on the overlapping portions where said metal electrodesoverlap with said transparent electrodes thereby to remove said a-Silayer at the lower portions of spaces between said metal electrodepatterns and to crystallize said a-Si layer at said overlappingportions, which contact the removed portions.
 2. The method of forming athin film solar cell having increased durability to high temperaturesand high humidity, of claim 10, including the steps of:placing a layerof a transparent electrically conductive film on a glass substrate;forming transparent electrodes with a beam of a YAG laser light having awavelength of about 1.06 μm and a spot diameter of 60 μm; applying anapproximately 1 μm thick amorphous silicon layer having a p-i-n junctionstructure by plasma CVD; and, applying and hardening metal electrodepatterns by printing.